1. Field Of The Invention
The present invention relates to a control device for a vehicle generator.
2. Discussion Of Background
As a control device for a vehicle generator there has been known the one which is disclosed in JP-A-62-104500(U.S. Pat. No. 4,739,243). In FIG. 2, there is shown the structure of the conventional control device for a vehicle generator. In this Figure, reference numeral 1 designates an a.c. generator which is driven by an internal combustion engine not shown, and which comprises armature windings 101 and a field winding 102. Reference numeral 2 designates a rectifier which rectifies the output from the a.c. generator in a full-wave, and which comprises a main output terminal 201, a voltage detecting output terminal 202 and a ground (earth) terminal 203. Reference numeral 3 designates a voltage regulator which adjusts the output from the a.c. generator 1 at a predetermined value, and whose structure and operation will be described later on. Reference numeral 4 designates a battery. Reference numeral 5 represents various electrical loads of the vehicle. Reference numeral 6 designates a key switch. Reference numeral 7 designates an initially exciting resistor for initially exciting the field winding 102 of the a.c. generator 1. Reference numeral 8 designates a reverse current blocking diode.
Now, the structure and operation of the voltage regulator 3 will be explained. Reference numerals 301 and 302 designate potential dividing resistors which divide the output from the voltage detecting output terminal 202 of the rectifier 2. When the divided potential is not less than the predetermined value, it causes a Zener diode 303 to conduct, allowing a transistor 304 to conduct. By conduction of the transistor 304, a transistor 305 for turning on and off the field winding 102 is driven to nonconduction to cut off a field current, thereby decreasing the output of the generator 1. When the divided potential by the potential dividing resistors 301 and 302 becomes lower than the predetermined value because of a decrease in the output of the generator 1, the Zener diode 303 is driven to nonconduction, thereby driving the transistor 304 to nonconduction as well. It causes the transistor 305 to conduct, allowing the field current to flow and the output of the generator 1 to be raised. The output from the generator 1 is adjusted so as to keep the predetermined value by repeating such operations.
When the field current flows in the field winding 102, a voltage drop is developed across a resistor 308 as a field current detecting element in proportion to the magnitude of the field current. The potential generated by the voltage drop is introduced into a comparator 310 as field current limiting means where the potential is compared with a reference voltage determined by dividing the output of a constant voltage source (not shown) by voltage dividing resistors 311 and 312. When the field current is small, the comparator 310 provides an output at a low level to have no effect on the voltage regulator 3. On the other hand, when the field current is great, the comparator 310 provides an output at a high level to flow a base current in the transistor 304 through resistors 315 and 316, and a reverse current blocking diode 317. It causes the transistor 304 to conduct and drive the transistor 305 to nonconduction, thereby cutting off the field current to decrease the output from the generator 1. When the field current is cut off, a discharge current flows from a capacitor 313, depending on a time constant determined by the capacitor 313 and a resistor 314, so the potential gradually lowers. When the potential becomes not higher than the reference voltage, the comparator 310 provides an output at a low level to cut off the base current for the transistor 304. It drives the transistor 304 to nonconduction, and it causes the transistor 305 to conduct, allowing the field current to flow so as to increase the output of the generator 1. In that manner, limiting the field current is carried out.
Usually, the voltage regulator 3 as stated earlier is constituted by a hybrid integrated circuit. As shown in FIG. 3, the transistor 305, a surge absorbing diode 307 and the field current detecting resistor 308 are constituted as discrete elements, and other parts are constituted as an IC. The voltage regulator includes a power source terminal "a", a field current controlling terminal "b" and a ground (earth) terminal "c" as terminals for external connection. The field current detecting resistor 308 and the IC have respective ground (earth) patterns coupled together at a terminal portion of the ground terminal "c".
In the conventional control device for a vehicle generator thus constructed, lead wires for taking out a signal from the hybrid integrated circuit are usually coupled by soldering. Since coupling by soldering causes contact resistance, there is provided the same situation as respective resistors are included in series between the field current detecting resistor 308 and the ground terminal "c", and between the voltage dividing resistor 312 and the ground terminal "c". In addition, the ground pattern of the field current detecting resistor 308 and the ground pattern of the IC are coupled at the terminal portion of the ground terminal "c" for external connection. This arrangement creates a problem in that each contact resistance varies depending on what soldering is made at the terminal portion, the field current detecting resistor 308 and the voltage dividing resistor 312 are unbalanced to make the ground potential for detecting the field current unstable, and limiting the field current is not carried out in an adequate manner unlike the operation of the field current limiting means stated earlier, thereby disturbing the control of the generator output, extremely raising the generator output and damaging an electrical system due to overcharge or overvoltage of the battery.